You should still get "5V" even using USB. At least, I do with my "2009".But, +V_usb has a wide range, too, so that could throw your readings off. Is that it?

"You gotta fight - for your right - to party!"Don't react - Read."Who is like unto the beast? who is able to make war with him?"It's "bipolar transistor" or "junction transistor" - "BJT" is just stupid.When all else fails, check your wiring!